ISIS Report 29/01/04
Methyl Bromide Ban
Prof. Peter Saunders
gives timely warning for maintaining the ban on a powerful ozone depleter
Six years ago, 160 countries agreed in the Montreal Protocol to phase
out substances that are known to deplete the ozone layer. The most important of
these were the chlorofluorocarbons (CFCs), which were being extensively used as
refrigerants and as propellants in aerosols. Also included was a plant
pesticide, methyl bromide, which developed countries are supposed to have
phased out by 2005. Developing countries were allowed a further ten years.
Methyl bromide (CH3Br) is widely used for fumigating
agricultural products (including wood) and to kill the organisms in soil in
which crops such as tomatoes and strawberries are to be planted. It tends to be
used more in warmer climates where soft fruits are more susceptible to attack.
When methyl bromide reaches the stratosphere it is split by ultra-violet
radiation. This releases bromine, which is approximately 40 times as effective
as chlorine at breaking down ozone. Only about a fifth of the methyl bromide
that enters the atmosphere comes from human activity, but it is such a powerful
ozone depleter that banning it makes an important contribution to restoring the
ozone layer. It is also highly toxic, and because it is volatile at 20 C, it is
hard to prevent it from drifting beyond the fields on which it is being applied
or to escape from greenhouses. When it enters the soil, it tends to penetrate
to the deeper soil layers where it breaks down only slowly, which makes it
dangerous to use in water collection areas.
Methyl bromide persists for less than a year in the atmosphere, so
measures taken now can have an immediate effect. Following the signing of the
Montreal Protocol, production of methyl bromide has declined and so has the
level in the atmosphere.
The Montreal Protocol allows exemptions for "critical users". This is a
category that was intended to allow banned substances to be used in small
quantities in applications in which there is no suitable alternative: CFCs are
still permitted as propellants in inhalers for asthmatics but the amounts
involved are relatively small. There is now, however, pressure to grant
large-scale critical use exemptions to farmers, especially in the USA, and so
that raw wood can still be used in packaging. If these are allowed, they could
halt, or even reverse, the decline in atmospheric methyl bromide.
A decision was expected in November but the delegates to the meeting at
the Nairobi headquarters of the United Nations Environment Programme (UNEP)
agreed they needed more time to consider the issues. There is now to be an
extraordinary meeting in Montreal in March 2004. With only a year to go before
the ban is meant to come into force, any users who do not gain the exemptions
they are hoping for will have very little time to comply. So its likely
to be a very difficult meeting.
Montzka, S.A., J.H. Butler, B.D. Hall, D.J. Mondeel, and J.W. Elkins, A
decline in tropospheric bromine, Geophysical Research Letters, 30 (2003)
Further reading can be found on the following web sites:
US Environmental Protection Agency
Methyl Bromide Alternatives Outreach www.mbao.org/